Small glass beads, or microspheres, have traditionally been used in a variety of applications. In one widely-used application, the microspheres are added to any of a variety of polymer compositions, with these compositions benefiting from the inclusion of the beads in any of a number of ways. For example, they may be used as reinforcement fillers for both thermoplastics and thermosetting resins to improve physical and thermal properties, reduce cost, and solve production problems. The spherical shape and non-porous morphology provide certain advantages when used in such applications. Incorporation of spherical glass particles increases the flow rate of polymers during forming operations, much as though the spheres are acting as tiny ball bearings. The non-porous and isotropic qualities of the microspheres may allow use of higher filler loadings, thereby reducing cost, and may further minimize the viscous drag between particles and allow for uniform and controllable shrinkage of molded parts during the cooling cycle. Typically, this helps to achieve formation of a smooth surface finish on the resulting parts.
Other particulate fillers, such as calcium carbonate, talc, and titanium dioxide may be used in polymer composites, coatings, and other formulation where whitening of the finished product is desired. However, many such fillers have irregular shapes and high surface areas that result in impeded flow during molding or other forming operations, thereby making such forming operations more difficult. It would be desirable to be able to use fillers that whiten compositions containing them, and provide the advantages afforded by spherical particles.
The particle size characteristics of glass microspheres can greatly influence their properties. For example, when the glass microspheres are to be used as a filler in a paint composition, the average size of the microspheres may affect viscosity and binder demand, the size distribution may affect packing and the resultant density and integrity of the paint film, and the top size may affect the gloss, sheen or smoothness of the paint as well as the tendency of the paint film to exhibit fracture failures or cracking.
Conventional production of glass microspheres typically involves comminution of glass (by milling, crushing and/or grinding) to provide irregularly-shaped glass particles that are then converted into spheroid form by feeding the particles into a vertical flame furnace which is maintained at a temperature effective to melt the glass particles. However, the glass particles tend to agglomerate as the particle size becomes small, resulting in the production of fused microspheres. Accordingly, it has been difficult to obtain glass microspheres which are fine and have a uniform particle size. In addition, the comminution step is laborious and energy-intensive. It would therefore be desirable to more directly obtain inorganic microspheres having a controlled, relatively narrow particle size distribution.